The present invention generally relates to cartridge filters for a pulse-jet industrial baghouse. More specifically, the invention relates to a unitary cartridge filter with an aerodynamic flow transition mouth for significantly improved filtering and cleaning performance.
Continuous emphasis on environmental quality has resulted in increasingly strenuous regulatory controls on industrial emissions. One technique which has proven highly effective in controlling air pollution has been separation of undesirable particulate matter from a gas stream by fabric filtration. Such filtration is carried out in dust collection apparatus known in the trade as a “baghouse” which operates on the same general principle as an ordinary household vacuum cleaner, except on a much larger scale. Basically, the baghouse is a sheet metal housing divided into two chambers, referred to as plenums, by a tube sheet. Disposed within uniform openings in the tube sheet are elongate filters. A particulate laden gas stream induced by the action of a fan, flows into one chamber (dirty air plenum) wherein dust accumulates on the filter media as the gas passes through the filter into the other plenum (clean air plenum) and out an exhaust.
Although all baghouses are designed in accordance with the foregoing general principles, there are numerous operational and structural distinctions including numerous differences in the construction of the filters themselves. The present invention relates to a baghouse with uniformly sized openings in the tube sheet adapted for use with cartridge filters.
A modern cartridge filter characteristically includes a central structural tube or core manufactured from a foraminous plastic or metal material. Circumscribing the central core is a ring of pleated filter media. A molded bottom end cap terminates the lower ends of both the core and filter media, commonly referred to as the pleat pack. The upper end of the pleat pack is terminated with an open mouthed, molded fitting sealed in one of several manners with the tube sheet. Filtration of the process gas therefore occurs from outside to inside of the filters (i.e., the dust collects on the outside surface of the pleated filter media) as the gas passes through the filter media, through the tubular core material and up to the dirty air plenum.
During continuous operation of the baghouse, the filters must be periodically cleaned and the accumulated dust removed. In a pulse-jet baghouse, cleaning is accomplished by delivering a short blast of high pressure air into the mouth of an individual filter cartridge. The higher pressure cleaning air temporarily overcomes the normal flow of process gas through the filter and travels the length of the cartridge to dislodge the dust cake accumulated on the outside of the filter pleats. The dust cake falls from the filter pleats to the hopper shaped bottom of the dirty air plenum for removal by an auger or similar means. At this point, the momentary effect of the short blast of cleaning air has dissipated and the normal flow of process gas though the cartridge filter resumes.
One of the earliest developments in a unitary cartridge filter is taught in my U.S. Pat. No. 5,632,791 showing a pleated filter with a closed bottom and a molded top configured with a peripheral exterior groove to sealingly mate with the circular opening through the tube sheet of the baghouse. The molding material was preferably a urethane having a durometer of between 30 to 70 shore A. Also important was a dimensional limitation to satisfy the relationship of 0.3<H/D<0.85 where D was the diameter of the tube sheet hole and H was the distance between the peripheral groove and the top of the pleat pack encased in the molding material. The foregoing two features, in combination, permitted sufficient resiliency for the upper portion of the top fitting to deform during installation or removal of the filter in the tube sheet and also to provide a reasonably effective seal between the peripheral groove and the tube sheet opening. The effectiveness of that seal could be further improved however by using a metal snap band positioned inside the mouth of the filter to push outwardly on the top fitting and to urge the peripheral groove to tighter engagement with the tube sheet. Such filters came to be known in the industry as “stepped-top” filters.
While the earliest unitary filter cartridges were formed as cylindrical constructions, eventually the need arose to adapt the features of unitary filter cartridges to non-round shapes in baghouses having oval or oblong openings. My patent U.S. Pat. No. 5,730,766 generally adapted the foregoing principles of stepped-top filter construction to a filter for an oblong opening. Due to the cross sectional length of the opening, here a snap band was mandatory in order to maintain an effective seal between the peripheral sealing groove of the top fitting and the tube sheet of the baghouse.
In U.S. Pat. No. 6,299,662, Poulsen used a metal expander in place of a snap band to achieve greater elastic compression of the upper collar of a stepped-top cylindrical filter cartridge for sealing against the tube sheet to accommodate variances in uniform circular hole size in the tube sheet openings. While achieving an improved seal, the expander naturally increased the expense associated with the filter cartridges and required a separate, specially manufactured part. Additional examples of a similar approach to the sealing problem are shown in U.S. Pat. No. 6,726,735 of Oussoren et al in which various configurations of the molded top were used in conjunction with an expander inserted into a stepped-top filter to try to achieve a more effective seal between the filter cartridge and the tube sheet opening. Obviously any leaks at all in this critical area impaired the filtering operation.
One source of such leaks was inherent in the molding process used in forming the top fitting of the filter cartridge. In order to mold the undercut surfaces forming the peripheral sealing groove in the top fitting, a two part mold was required. This left two external parting lines running vertically at opposite ends of the filter diameter caused by the halves of the mold forming the top fitting. This problem was exacerbated significantly with any mismatch between the mold halves during the manufacturing process. Such imperfections in the finished filter cartridge render sealing to the tube sheet very problematic.
One approach to overcome sealing issues is to provide a separate seal to the filter cartridge itself. Both my U.S. Pat. Nos. 6,858,052 and 7,186,284 provide examples of this approach. The sealing gasket is formed of a resilient elastomeric and is fitted into the tube sheet hole. The filter cartridge is then inserted through the central bore of the gasket to provide the necessary force to urge the gasket to engagement with the tube sheet. Now, instead of the top fitting portion of the filter cartridge being resiliently deformable, the top portion was required to be rigid in order to compress the sealing gasket. This was achieved by greatly reducing the H/D ratio such that the upper end of the pleat pack extended almost to the top flange of the cartridge to be positioned adjacent the tube sheet when the filter was installed. While providing the necessary rigidity to the filter, some drawbacks did exist with this solution. The interior thickness of the molding material had to be increased to sufficiently encase the pleat pack which, in turn, reduced the bore of the filter mouth. In addition, shrinking of the molding material caused extreme dimensional variations in the molded top which resulted in leakage problems.
On the positive side, as shown particularly in U.S. Pat. No. 7,186,284, an undercut surface to form a peripheral sealing groove was no longer necessary and the exterior side wall of the top fitting could thus be molded without undercut in a one piece mold to eliminate vertical parting lines in the finished product. With this development, however, a new problem arose. During the curing process, the molding material between successive pleats in the pleat pack would shrink slightly so that the outside surface of the top fitting had more of a scalloped configuration around the edges of the pleats than a smooth cylindrical surface. This represented potential leakage regions in situations where the filter cartridge, sealing gasket and hole size of the tube sheet were outside normal tolerances.
In the cleaning of baghouse filters, numerous structural and operational differences have been examined in an effort to improve efficiency. As these relate to the background of the present invention, many enhancements to the cleaning cycle have been proposed. Commonly accepted wisdom suggests that the use of a venturi in conjunction with pulse-jet technology wherein a short blast of high pressure air is delivered to the mouth of an individual filter cartridge improves cleaning efficiency. The intended effect of the venturi structure is to entrain aspirated or secondary air with the initial blast of high pressure cleaning air blast to overcome the normal flow of process gas through the filter and to travel the length of the cartridge to dislodge the dust cake accumulated on the outside of the filter pleats. Effective cleaning power is achieved by a combination of two physical characteristics—that is, fluid flow through the filter media and pressure drop across the filter media. A venturi is known to increase the fluid flow during the cleaning cycle which improves cleaning performance. However, it is also know that the venturi structure itself increases pressure drop across the filter during normal filtering operation and this is a deleterious effect to the overall baghouse operation since any subsequent cleaning cycle must overcome the overall pressure differential of the filter cartridge itself, the accumulated dust cake, the opposing process gas flow and the physical characteristics of the equipment involved such as the venturi itself.
Accordingly, in spite of the advancements made to date in the filtering arts, a need remains for a highly efficient unitary cartridge filter which minimizes pressure drop for both filtering and cleaning cycles and which maximizes cleaning power over a range of operating conditions. There is also a need for a high efficiency unitary cartridge filter to effectively seal with circular and oblong sized tube sheet holes within manufacturing tolerances in order to minimize leakage of process gas from the dirty air plenum to the clean air plenum. The primary objectives of this invention are to meet these diverse needs.